1. Field of the Invention
This invention relates to a method for forming oriented metallic nanowires, more particularly to a method involving growing metallic nanowires by reduction reaction of metal salt solutions in a downward direction toward earth's gravity.
2. Description of the Related Art
Conventional methods for forming metallic nanowires include the template synthesis method, the seed-mediated growth method, and the thermal-induced forming method.
In the template synthesis method, a porous material, such as anodic alumina membrane (AAM), SiO2 and zeolite, formed with a plurality of nano-sized pores therein is used as a template such that metallic nanowires are grown in the nano-sized pores. Formation of the metallic nanowires in the nano-sized pores can be conducted through soft chemical solution, sol-gel or electroplating techniques. The template synthesis method is disadvantageous in that the cost of the template is relatively expensive. In addition, the number and length of the metallic nanowires thus formed are limited by the nanostructure of the template.
In the seed-mediated growth method (Adv. Mater. 2002, 14, No. 1, January 4, p. 80˜82), metal ions in a metal salt solution are reduced using a strong reducing agent so as to form seeds, i.e., nucleation sites, each having a diameter of 3-5 nm. The metal salt solution containing the seeds is then added into another solution, which contains a high concentration of the metal salt, a weak reducing agent and a rodlike micellar template, for subsequent steps of forming the metallic nanowires. However, the seed-mediated growth method is relatively complex and has a poor control in growing the metallic nanowires.
In the thermal-induced forming method (J. Vac. Sci Technol. B 23 (6), November/December 2005, p. 2261˜2265), a TiO2 film is formed on a Si substrate, followed by applying a metal salt solution, such as AgNO3 solution, on the anatase-TiO2 film. Electrons and electron holes are formed on a surface of TiO2 films by exposure to ultraviolet (UV) light. The metal ions are trapped by the excited electrons, which results in the reduction of metal ions to atoms. Heating at 300° C.-500° C. provides thermal energy to overcome the energy barrier to allow reduction metal ions and accumulation of atoms. Although the thermal-induced forming method is simple and cost effective, the metallic nanowires thus formed are randomly oriented.